Structural and Sorption Characteristics of Nanocellulose Aerogel-Based Composite Materials Doped with Mefenamic and Flufenamic Acids: Insights from Magic Angle Spinning and High-Pressure NMR Spectroscopy

Our research, which employs a comprehensive approach combining high-pressure and magic angle spinning (MAS) NMR spectroscopy methods, investigates the structural and sorption characteristics of composite materials based on cellulose aerogels. The ¹³C NMR studies, conducted in supercritical CO₂, revealed kinetic parameters for the sorption processes that significantly differ from those of silica-based analogs. The analysis of chemical shifts from ¹⁹F MAS NMR spectra led to the first identification of two stable phase states of flufenamic acid within the cellulose aerogel: an amorphous phase within the pore volume (-62.15 ppm) and a liquid-like phase on the surface of the pores (-65.09 ppm), with a ratio of 2:1. These findings, which contrast starkly with previous data for silica systems, where only a liquid-like state was observed, underscore the unique three-dimensional architecture and highly developed porous structure of cellulose aerogels. These features appear to stabilize metastable amorphous phases of active pharmaceutical ingredients (APIs), opening up new perspectives for the use of cellulose aerogels in the controlled stabilization of metastable phases of APIs in composite materials.